Modern disk drive designs typically embed a certain amount of overhead information, such as servo head position information and data identification fields for example, within the concentric data storage tracks of a disk drive. Usually, this embedded information is recorded in evenly spaced apart areas or "sectors" of the track, and in some cases interrupts fixed-length user data blocks otherwise recorded in the tracks. The embedded servo information typically includes servo head position and track/data identification fields, and also typically includes a unique servo address mark pattern which is provided to enable resynchronization of timers for recovering the servo head position and the track/data identification field information, and which mark in time expected arrival of the next embedded servo sector. Reliable and robust detection of the servo address mark pattern in each servo sector is essential for precisely marking in time not only the servo sector and its head positioning information fields, but also the following user data storage area.
When embedded overhead information is present within the storage track, a selected head following the track will alternately switch between reading or writing user data, and reading the overhead information. Because of the desire to increase data storage density, the guard bands and gaps heretofore provided between user data and embedded overhead information are continually being narrowed to reduce embedded overhead, and the active user data storage area is being placed closer and closer to the overhead information, in order to gain further user data storage capacity within the limited area of the data storage surface.
One drawback associated with modern data transducer structures occurs during a switch-over from writing to reading. Typically, switchover occurs when a write-enable control line becomes false. When this abrupt switch-over edge occurs, writing current is suddenly removed from the head structure, leaving behind marginally stable magnetic domain patterns within the head structure, particularly the poles and pole tips. These marginally stable magnetic domain patterns may subsequently rearrange themselves into more stable patterns, and in the process of doing so release energy, as the magnetic structure relaxes following writing. The released energy within the head structure becomes manifested electrically as electrical voltage noise spikes known in the art as Barkhausen noise or "popcorn" noise. Barkhausen noise spikes create spurious voltage spikes in the analog electrical signal stream present in the read channel from the head. While Barkhausen noise events may occur in ferrite heads, metal-in-gap heads, and thin film inductive heads, it is very prevalent in thin film inductive head structures. A scientific investigation of Barkhausen noise phenomena is reported upon by K. B. Klassen and JCL. von Peppen in "Barkhausen Noise in Thin-Film Recording Heads", IEEE Trans. on Magnetics, Vol. 26, No. 5, Sep. 1990, pp. 1697-1699.
Since the spikes occur following a switchover from writing to reading as at a servo sector, the spikes may corrupt the embedded overhead data including a servo address mark. If a servo address mark is corrupted or misdetected, subsequent events may become mistimed leading to possible overwrite of a servo sector or mis-synchronization to, or misdetection of, the servo and/or user data. The data corruption resulting from random Barkhausen noise events is not predictable, because the occurrence or precise location of the Barkhausen noise event relative to the switch-over point is unpredictable.
There have been several attempts to minimize the impact of Barkhausen noise upon disk drive operations. Depending upon the predicted location of the write-to-read transition, error correction techniques may be employed to remove the Barkhausen noise corruption of ensuing data being read back. One prior approach calls for tapering off the write current in a controlled fashion, such as in a decaying exponential curve, in an attempt to control magnetic recording head relaxation following a writing operation in a magnetic recording system, such as a disk drive. One example of this approach is given in U.S. Pat. No. 5,168,395 to Klassen et al., entitled: "Controlled Magnetic Recording Head Relaxation in a Magnetic Recording System." One readily apparent drawback of the Klassen et al. approach is that it requires further overhead space on the disk surface during the time interval following switchover from write mode to read mode when the head's magnetic structure is undergoing controlled relaxation.
Another approach, described in copending, commonly assigned U.S. patent application Ser. No. 08/137,807 by Gold, entitled: "Data Block Sequencing Using ID After Embedded Servo Sector in Disk Drive", called for placing the user data block ID field in a position immediately following each embedded servo sector. Thus, if a Barkhausen noise event occurred following switchover from writing to reading at a servo sector, the event would most likely occur before user data was again reached following the servo sector information and the immediately following user data block ID information. While this approach resulted in greater user data integrity than before, a problem of a Barkhausen noise event occurring during the servo address mark field remained unresolved.
Thus, a hitherto unsolved need has remained for a way of increasing data storage capacity by reducing the overhead space, while still permitting effective disk drive operations to recover embedded servo address marks even in the presence of some Barkhausen noise events.